1. Field of the Invention
The present invention relates to a flow control valve which is changed to and held in an opened state by pressure of compressed air against an urging force of a spring and more particularly to a flow control valve for use in a semiconductor manufacturing device.
2. Description of Related Art
As one of conventional flow control valves, for example, there is a flow control valve disclosed in Japanese unexamined patent publication No. 7(1995)-253170. FIG. 4 is a sectional view of such flow control valve 100. This flow control valve 100 has an under body 120 with an inlet port 121 and an outlet port 122 formed on either side. An intermediate body 130 formed with a first operation port 131 is secured on the under body 120. An upper body 140 formed with a second operation port 141 and attached with an adjustment screw 142 is secured on the intermediate body 130. Thus, the outer shape of the whole flow control valve 100 is determined.
The under body 120 is centrally provided with an annular valve seat 101 in addition to the inlet port 121 formed at a right side in the figure and the outlet port 122 formed at a left side in the figure. An inside A of the valve seat 101 is communicated with the inlet port 121, whereas an outside B of the valve seat 101 is communicated with the outlet port 122. When a valve body 102 is brought into contact with the valve seat 101, communication between the inlet port 121 and the outlet port 122 is interrupted. When the valve body 102 is brought out of contact with the valve seat 101, the communication between both ports 121 and 122 is allowed.
The intermediate body 130 is a substantially cylindrical member secured on the under body 120 at a center area thereof. The intermediate body 130 is formed with the first operation port 131 and further internally formed with a small-diameter cylinder 132 and a large-diameter cylinder 133. In the intermediate body 130, a substantially columnar piston 150 is fit to be slidable up and down. The piston 150 has a central large-diameter portion 151, a lower small-diameter portion 152 provided under the large-diameter portion 151, and an upper small-diameter portion 153 provided above the large-diameter portion 151. The large-diameter portion 151 is air-tightly engaged in the large-diameter cylinder 133. The lower small-diameter portion 152 is air-tightly engaged in the small-cylinder 132. A lower surface 154 of the large-diameter portion 151 and the intermediate body 130 define a first operation chamber 134. The first operation port 131 of the intermediate body 130 opens into the first operation chamber 134. Through the first operation port 131, air pressure is applied to or released from the first operation chamber 134. When the air pressure is applied to the first operation chamber 134, the piston 150 is pushed up.
The valve body 102 is attached to a lower end of the lower small-diameter portion 152 of the piston 150. The circumferential portion of the valve body 102 forms a diaphragm 156 of which the peripheral edge is sandwiched between the under body 120 and the intermediate body 130. The valve body 102, which is movable in conjunction with the vertical movement of the piston 150, is brought out of contact or into contact with the valve seat 101 of the under body 120. When the valve body 102 is brought into contact with the valve seat 101, communication between the inlet port 121 and the outlet port 122 is interrupted. When the valve body 102 is brought out of contact with the valve seat 101, the communication between both ports 121 and 122 is allowed.
The upper body 140 is a substantially cylindrical member secured on the intermediate body 130. The upper body 140 is centrally formed with a through hole 143 in addition to the second operation port 141. The upper small-diameter portion 153 of the piston 150 is engaged in the hole 143 of the upper body 140. The upper surface 155 of the large-diameter portion 151, the intermediate body 130, and the upper body 140 define a second operation chamber 144. The second operation port 141 of the upper body 140 opens into the second operation chamber 144. Through the second operation port 141, air pressure is applied to or released from the second operation chamber 144. When the air pressure is applied to the second operation chamber 144, the piston 150 is pushed down. The upper body 140 is formed with a spring groove 145. A return spring 146 is located between the upper surface 155 of the piston 150 and the spring groove 145. The return spring 146 urges the piston 150 downward.
An upper half of the hole 143 of the upper body 140 is formed with screw threads. The adjustment screw 142 is mounted in this portion. The adjustment screw 142 has a lower end 147 which restricts upward movement of the piston 150. Changing of the height of the lower end 147 by turning of the adjustment screw 142 allows changes of a stop position of the piston 150, thereby adjusting a clearance between the valve body 102 and the valve seat 101 at a valve opening time. It is to be noted that a lock nut 148 may be used to hold the adjustment screw 142 against inadvertent movement.
The operation of the flow control valve 100 having the above structure will be explained below. The flow control valve 100 is operated by the application of air pressure to the first operation port 131 or the second operation port 141. A means for supplying the air pressure may be anything, for example, a compressed-air cylinder and an air-pressure pump.
The state where no air pressure is applied to both the first operation port 131 and second operation port 141 is first considered. In this state, the piston 150 receives the urging force of only the return spring 146. The piston 150 is therefore moved downward until the valve 102 attached at a lower end of the piston 150 comes into contact with the valve seat 101. In this state, the valve body 102 is held in contact with the valve seat 101, interrupting communication between the inlet port 121 and the outlet port 122, the flow control valve 100 is closed.
When the air pressure is applied to the first operation port 131, the first operation chamber 134 of the flow control valve 100 is increased in pressure. This pushes the piston 150 upward against the urging force of the return spring 146 until the upper end of the piston 150 comes into contact with the lower end 147 of the adjustment screw 142, and then the piston 150 is stopped. The valve body 102 is accordingly moved together with the piston 150, providing a clearance between the valve seat 101 and the valve body 102, thereby allowing the communication between the inlet port 121 and the outlet port 122. Thus, the flow control valve 100 becomes opened.
In this state, when the adjustment screw 142 is operated to change the position of the lower end 147, the stop position of the piston 150 is changed. The clearance between the valve seat 101 and the valve body 102 is adjusted to control a flow rate during an opened state of the flow control valve 100.
When supply of the air pressure to the first operation port 131 is stopped and the pressure of the first operation chamber 134 is released, the flow control valve 100 is placed in a closed state again by the urging force of the return spring 146. At that time, air pressure is applied to the second operation port 141, the pressure in the second operation chamber 144 becomes high. This pressure assists the urging force of the return spring 146 to push the piston 150 downward. A valve closing operation is further ensured.
The control of a flow rate by means of the flow control valve 100 in FIG. 4 is however performed by manual turning of the adjustment screw 142 to change the lower end 147 of the screw 142 with which the upper end of the piston 150 comes into contact, changing the stop position of the piston 150 in the opened state. Accordingly, it could not be conducted by remote and high-precision control.
Particularly, the control of a flow rate by remote and high-precision control is required for a semiconductor manufacturing device. On this account, the flow control valve 100 shown in FIG. 4 would be unusable in the semiconductor manufacturing device.
FIG. 5 shows the flow control valve 100 of FIG. 4 additionally attached with an electro-pneumatic regulator 160 which controls a normally-closed proportional intake valve 161 and a normally-closed proportional discharge valve 162 through a control substrate 163. In this flow control valve 100 of FIG. 5, air pressure is applied to or released from the first operation port 131 through the electro-pneumatic regulator 160 to bring the valve into the opened state or the closed state. Thus, when the normally-closed proportional intake valve 161 is opened and simultaneously the normally-closed discharge proportion valve 162 is closed, air pressure is supplied and applied to the first operation chamber 134, thereby bringing the valve into the opened state and holding it in that state. Upon de-energization in this state, however, the normally-closed proportional intake valve 161 and the normally-closed proportional discharge valve 162 are closed, but the air pressure in the first operation chamber 134 remains retained. Depending on the state or the case, the opened state is sustained. This may continuously cause an outflow of a fluid to be controlled.
Especially, if the outflow of the controlled fluid is continued during de-energization, the flow control valve 100 of FIG. 5 would be unusable in the semiconductor manufacturing device which requires accurate flow control.